The present invention relates to a process for preparing a foamed polymeric article comprising heating a polymeric composition, said composition comprising a thermoplastic polyolefin elastomer, a first blowing agent and a second blowing agent, to a temperature at which the first blowing agent releases water, to foam said thermoplastic polyolefin elastomer, said temperature being above the melting point of the thermoplastic polyolefin elastomer. The invention also relates to the polymeric composition and to a foamed article obtainable by the process of the invention.
A process is known from EP-A-40,934, in which a blend of a polyolefin resin and a rubber is foamed by the use of either chemical or physical blowing agents.
A drawback of the known process is that the proposed blowing agents (e.g. chemical blowing agents, (understood to be chemical compounds which decompose under heating, to liberate one or more gaseous components, such as azodicarbonamides), or physical blowing agents, (understood to be compounds, which for foaming purposes is heated to a temperature at which the compound is gaseous, such as chlorofluorocarbons or low boiling hydrocarbons)) have all their disadvantages, like inefficiency, and/or a detrimental influence on the ozon-layer of the earth-atmosphere. An overview of blowing agents can be found in xe2x80x9cHandbook of Polymeric foam and foam technology, (edt. by D. Klempner and K. C. Frisch; Hansen Publishers, 1991)xe2x80x9d, esp. chapter 17 (page 376-408).
It has been proposed to use gases, like N2 or CO2, as an alternative, but they are difficult to mix in thermoplastic elastomers.
In the prior art the use of liquid water as a blowing agent is known. WO-92/18326 as well as EP-A-503,220 describe a process for the preparation of foamed thermoplastic elastomers in which the thermoplastic polyolefin elastomer is heated to a temperature above its melting point, after which water is added to the melted thermoplastic polyolefin elastomer and the mixture is released to atmospheric pressure.
Such a process has many drawbacks: the liquid water has to be injected and mixed with the thermoplastic polyolefin elastomer; the process results in a non-uniform foam having large and irregular shape as well as surface roughness. Such a process also requires special equipment for the mixing of the ingredients and the metering of the water.
R. Niemark (xe2x80x9cNew low density TPE-V foams produced in conventional processing equipmentxe2x80x9d, at Foamplas xe2x80x297) indicates in his article the use of a chemical blowing agent in the form of a filler, whereby the chemical blowing agent decomposes when heated to release water.
The present invention provides a solution for the above problems. The present invention relates to a process for preparing a foamed polymeric article comprising heating a polymeric composition, said composition comprising a thermoplastic polyolefin elastomer, a first blowing agent and a second blowing agent, to a temperature at which the first blowing agent releases water, to foam said thermoplastic polyolefin elastomer, said temperature being above the melting point of the thermoplastic polyolefin elastomer, characterized in that said first blowing agent is a water releasing chemical compound selected from the group consisting of a metal hydroxide, a Group 1 or 2 of the Periodic Table (CRC Handbook of Chemistry and Physics, 1990) metal salt containing hydrate water or mixtures thereof, and said second blowing agent not being a metal hydroxide or a Group 1 or 2 of the Periodic Table metal salt containing hydrate water.
The invention also relates to a foamable polymeric composition; it also relates to a foamed polymeric article foamed from the foamable polymeric composition.
It has been found that specifically the combination of the at least one first and the at least one second blowing agent results in highly improved properties of the foams prepared by the process of the present invention, compared to the results of the known blowing agents.